Over the years, a number of techniques and apparatus have been utilized for securely attaching a prosthetic device to a bone. With respect to total or partial reconstruction of the hip joint, it is desirable to replicate a normal natural hip joint in which the head of the femoral bone that extends from the knee to the pelvis, articulates with the socket (acetabulum) of the hip bone. In such total or partial arthroplasty (surgery for formation of movable joints) it is critical that there be no loosening of the bond between a femoral head prosthesis and living femoral bone.
Known procedures and devices for arthroplasty of the hip include a femoral head prosthesis which is anchored to the shaft of the femoral bone, such as the one disclosed in an article entitled "Anchorage of the Femoral Head Prosthesis to the Shaft of the Femur" by John Charnley published in The Journal of Bone and Joint Surgery, Volume 42B, No. 1, February, 1960. In this article, a prosthesis has the shape of a femoral head with a metallic stem. The stem is shaped generally to conform to the internal shape of the femoral bone. In the process, the upper end of the femoral bone is hollowed out sufficiently to provide a cavity. The cavity is then filled with cement, and the stem of the prosthesis is inserted into the cavity to expand the cement and drive it into intimate contact with the internal shape of the bone. This device attempts to prevent loosening of the bond between the femoral head prosthesis and the living bone by providing a large area of contact between the prosthesis and the bone in order to distribute the load as widely as possible. An acrylic cement is used with this device purely as a filling and functions as though it were part of the prosthesis. In essence, the cement acts to fill voids between the prosthesis and the bone to make the combination function as though the stem of the prosthesis were cast in metal to the exact shape and size of the hollowed out cavity. Unfortunately, this technique involves removing large portions of tissue from the femoral canal. In addition, since it uses acrylic cement, it is possible for the cement to wear and loosen upon repeated contact of the weight bearing hip joint. Moreover, such a technique is fairly complicated and involves possible weakening of the bone due to such large tissue removal from the bone. This loss of bone substance makes revision arthroplasty still more complicated and risky.
Owing to complications related to loosening of the components of total hip arthroplasty, as a result of incompetence of the cement interface, uncemented prostheses were developed and then implanted around 1983. These were basically the same design as the cemented arthroplasty except fixation relied on an interference fit between the prosthesis and the medullary cortex of the femur. In addition, various amounts of a porous surface were added to the prosthesis which theoretically would allow for bony ingrowth to help anchor the implant (Morscher, E., "The Cementless Fixation of Hip Endoprostheses", Berlin, Springer-Verlag, 1984). The same problems of massive bone removal, violation of the femoral medullary canal, stress-shielding of the proximal femur (where the implant absorbs the stresses placed on the proximal femur and the unstimulated bone artophies to soft, weakened tissue prone to fracture) remain as well as the new risk of tumor, allergic reaction, and metal ion toxicity due to the increased surface area of metal from the porous surface. (Swensson, O. et al., "Formation of a Fulminant Soft-Tissue Pseudotumor after Uncemented Hip Arthroplasty", J. Bone Joint Surg., 70A, pp. 1238-1242, 1988; Buchert, P. K. et al., "Excessive Metal Release Due to Loosening and Fretting of Sintered Particles on Porous Coated Hip Prostheses", J. Bone Joint Surg., 68A, pp. pp. 606-609, 1986; and Brown, G. C. et al., "Sensitity to Metal as a Possible Cause of Sterile Loosening after Cobalt-Chromium Total Hip Replacement Arthroplasty", J. Bone Joint Surg. 59A, pp. 164-168, 1977). Uncemented implantation of a hip prosthesis has the advantage of avoiding cement fixation but has produced a set of its own unique problems.
The main problem with the described approaches to total hip arthroplasty has to do with the necessity to perform a revision when the patient outlives the implant. The young patient aged thirty to forty (30-40) years who undergoes total hip arthroplasty can anticipate a greater than fifty percent (50%) chance of having a loose prosthesis five (5) years after surgery (Dorr, L. D. et al., "Total Hip Arthroplasty in Patients Less Than 45 Years Old", J. Bone Joint Surg., 65A, pp. 474-479, 1983). Since each successive arthroplasty can be expected to fail earlier (Kavanaugh, B. F. et al., "Revision Total Hip Arthroplasty", J. Bone Joint Surg., 67A, pp. 517-526, 1985) and have a higher risk of infection with each successive operation (Hunter, G. A., "The Results of Revision Total Hip Arthroplasty", J. Bone Joint Surg., 61B, pp. 419-421, 1979), an implant which preserves bony stock (allowing easier revision) such as a cementless load-sharing device is desirable. Such a device which is also relatively risk free with respect to metal ion toxicity would be a useful and unique invention.
Other known techniques and designs are disclosed in an article entitled "Total Hip Articular Replacement by Internal Eccentric Shells" by H. C. Amstutz, et al., Clinical Orthopedics and Related Research, No. 128, October, 1977, pp. 261-284. It discloses a number of various design approaches for arthroplasty, including resurfacing the femoral head using cemented metal femoral cups. It also discloses a procedure called the "Tharies" approach, an acronym for Total Hip Articular Replacement by Internal Eccentric Shells. In accordance with the Tharies procedure, portions of the femoral head are removed, as by reaming, to provide notches and an overall shape for accepting a hollow metal femoral shell which is cemented onto the reamed femoral bone. This shell is then implanted in the hip bone of a matching acetabular shell. The device attempts to solve problems related to long term durability of conventional hip replacements including calcaresorbtion, acrylic cracking, prosthetic loosening, and stem fractures. Unfortunately, this technique requires implantation of an acetabular shell in the hip bone. In addition, since acrylic cement is involved, there are problems associated with loosening which may occur whenever a cement-type bonding technique is used.
Further attempts have been made to improve the "Tharies" technique by utilizing a system incorporating a stem-type femoral head prosthesis in combination with matching internal eccentric shells. Known attempts to improve the holding capability have included using standoff pegs to reduce the potential for the stem to bottom out in the cavity which is excised into the femoral bone, and using special coatings on the stem to improve bonding of the cement to the stem.
The present invention recognizes the need for a simple yet effective technique and apparatus for securely anchoring a prosthesis on the head of a femoral bone for use in a joint. The present invention further recognizes the need for an apparatus which firmly holds the prosthesis to the living bone to prevent progressive loosening and failure of the fixation, while avoiding violation of the internal bone canal. The present invention further recognizes the advantages of minimal bone resection, i.e. excision of a portion of the bone. In addition, the present invention is useful as an endoprosthesis alone, or can be utilized in a total hip arthroplasty.
Accordingly, it is an object of the present invention to provide an implant for use as a body joint element which is firmly fixed to the living bone. It is a further object of the present invention to provide an implant which is capable of effectively carrying the load presented on the joint. It is yet another object of the present invention to provide a prosthesis which may be implanted with minimal bone resection. Another object of the present invention is to provide a method and apparatus for simple and efficient implantation of a femoral head prosthesis. Yet another object of the present invention is to provide an implant which is reliable and durable. Still another object of the present invention is to provide a prosthesis which is relatively easy to manufacture and comparatively cost-effective. An additional object of the present invention is to increase the ease with which this prosthesis can be revised to a conventional total hip arthroplasty in the face of the patient outliving the implant.